Detected impacts of atmospheric rivers on marine heatwaves.

Marine heatwaves (MHWs) are periods of unusually high sea surface temperature that can persist for weeks to months and extend across thousands of kilometers. Their increasing frequency and intensity under climate change threaten marine ecosystems and fisheries, yet the physical processes that govern their occurrence and evolution remain poorly understood. Here we analyze satellite and reanalysis data to show that atmospheric rivers (ARs)-long, narrow corridors of concentrated atmospheric moisture, often described as "rivers in the sky"-play a previously overlooked role in the development of MHWs in the North Pacific and North Atlantic. Under an AR, increased cloud cover cools the ocean through reduced solar radiation, while anomalously warm, humid air warms the ocean through the reduction of turbulent heat fluxes from the ocean. These two opposing mechanisms, dominant among others, vary with background climate state, causing seasonally and regionally varying ARs' impacts on MHWs. These findings stress the importance of understanding ocean-atmosphere compound extremes and their changes under a warming climate.